- (BOOL)respondsToSelector:(SEL)aSelector
Returns a Boolean value that indicates whether the receiver implements or inherits a method that can respond to a specified message. (required)
Paraments:aSelector: A selector that identifies a message.
Return:YES if the receiver implements or inherits a method that can respond to aSelector, otherwise NO.
判断是否实现了方法。
http://www.cnblogs.com/chenjunbiao/archive/2011/04/21/2023196.html